Sensor elements for ascertaining at least one proportion of a gas in a gas mixture are known from the related art. Without limiting other possible embodiments, the present invention is described in the following with reference to devices used for quantitatively and/or qualitatively ascertaining at least one proportion, in particular of a partial pressure and/or a volume fraction and/or a mass fraction of a gas in a gas mixture. The gas may be an exhaust gas of an internal combustion engine, for example, in particular in automotive applications.
Most notably, a lambda probe is used as sensor element for ascertaining the gas proportion. Lambda probes are described, for example, in Konrad Reif (publisher): Sensoren im Kraftfahrzeug (Sensors in the Motor Vehicle), 2nd edition, 2012, pp. 160-165. Different variants of lambda probes are known from the related art.
These include lambda probes having a cell that are also referred to as “two-point lambda probes.” The two-point lambda probes compare the residual oxygen proportion in the exhaust gas to the oxygen proportion of a reference gas atmosphere that can be present as circulating air inside of the sensor device, and indicate whether there is a rich mixture (i.e., lambda<1) or a lean mixture (i.e., lambda>1) in the exhaust gas. In the lambda probe that includes a cell, an external electrode is in contact with a gas chamber having a high oxygen concentration, preferably a reference volume. A fixed voltage is applied between the external electrode and an internal electrode of the cell. As soon as an oxygen concentration in a cavity approaches 0, a Nernst potential rises sharply and partially compensates for the applied voltage. This makes it possible to adjust and maintain a constant oxygen concentration in the cavity with a high degree of accuracy.
For various reasons, it may be advantageous to know the internal resistance of the sensor element, particularly since the internal resistance of the sensor element influences different properties of the sensor element and/or an engine management system that accesses quantities measured by the sensor element. To be mentioned by way of example in this context are electrical diagnoses of the sensor element, recognizing an operational readiness of the sensor element, and stabilizing the temperature of the sensor element.
To determine the internal resistance of the sensor element, it is conventionally provided that a current pulse be applied to the sensor element. In this case, the “current pulse,” also referred to as a “measuring pulse,” is understood, in particular, to be a sharp rise in the current that flows through the first electrode, the solid electrolyte connecting the first electrode and the second electrode, and through the second electrode of the sensor element. A resultant current loading of the sensor element brings about a charge transfer in the sensor element; the charge transfer in the sensor element being able to induce an increase in an electrical voltage present between the first electrode and the second electrode. A value for increasing the electrical voltage in response to the application of the current pulse to the sensor element can be ascertained from the observed variation of the electrical voltage between the first electrode and the second electrode immediately thereafter.
Conventionally, the internal resistance of the sensor element can be determined in each particular case by correlating the voltage between the first electrode and the second electrode of the sensor element with the described current loading to without the described current loading. However, applying the current pulse to the sensor element also leads to the above described charge transfer being induced in the sensor element. Since the cell in the sensor element also always has a capacitive portion, the current pulse can thus lead to an additional increase in the voltage present at the cell. This is known to one skilled in the art from a charging or discharging of a capacitor. However, this additional increase in voltage in the cell can lead to a deviation between the value defined for the internal resistance of the sensor element and the actual value for the internal resistance of the sensor.
For this reason, it is advantageous to record the characteristic of the electrical voltage between the first electrode and the second electrode of the sensor element under current loading preferably shortly after the current load is applied to the sensor element. In practice, however, it is not possible to implement this advantageous procedure since the sensor element, in particular the lambda probe, is typically connected via a low-pass filter to the corresponding engine management system, especially to largely suppress a transmission of high frequency signal interference from the engine management system to the sensor element. Therefore, in practice, the electrical voltage present between the first electrode and the second electrode under current loading is typically recorded only three milliseconds after the current loading begins. Although the charge transfer in the sensor element that occurs during these three milliseconds can influence the result obtained upon determining the internal resistance of the sensor element, this effect is conventionally ignored when the internal resistance is ascertained.